Road traffic lights responding automatically to the arrival and presence of vehicles have long been known and were originally operated by the passage of the approaching vehicles over pressure switches installed in the road across the traffic lanes. Those suffered from the disadvantage of being subjected to great wear and tear and the possibility of serious damage from the vehicles passing over, as well as being difficult to protect on a long-term basis against weather and the ingress of water and dirt. They were therefore superseded by the present-day systems in which the detector device responding to the arrival and presence of a vehicle is an inductive electrical loop buried in the road. But these have brought their own set of problems, notably susceptibility to environmental drift and cross-talk between loops installed in proximity to one another, with the result that the electronic networks employed for evaluating the signals from such vehicle-presence-detector loops have become increasingly complex. It is therefore an object of the present invention to provide a new approach to road vehicle presence detection.